The use of acoustic (e.g., audible and/or ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications, is well known. Such acoustic measurement systems are utilized in a variety of downhole applications including, for example, borehole caliper measurements, measurement of drilling fluid properties, and the determination of various physical properties of a formation. In one application, acoustic waveforms may be generated at one or more transmitters deployed in the borehole. The acoustic responses may then be received at an array of longitudinally spaced apart receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred to as slownesses) of a formation.
It will be appreciated that the terms slowness and velocity are often used interchangeably in the art. They will likewise be used interchangeably herein with the understanding that they are inversely related to one another and that the measurement of either may be converted to the other by simple and known mathematical calculations. Additionally, as used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) whereas LWD typically refers to measurements taken for the purpose of analysis of the formation and surrounding borehole conditions. Nevertheless, these terms are herein used synonymously and interchangeably.
In the analysis of acoustic logging measurements, the received acoustic waveforms are typically coherence processed to obtain semblance data which may be displayed on a time-slowness plot. In a time-slowness plot, also referred to as a slowness-time-coherence (STC) plot or a semblance plot, a set of several signals from the array of acoustic receivers is processed with the incorporation of separate time shifts for each received signal. The separate time shifts are based on a slowness value assumed for the purpose of processing the waveforms. The processing provides a result, known as coherence, which can signify the presence of a discernable signal received by the separate receivers. In this manner compressional and shear wave arrivals can be discerned in the received waveforms. One well known problem with this technique is that aliasing of the compressional arrival often interferes with a shear wave arrival. This aliasing effect may mask or mimic the presence of a shear wave signal and tends to be particularly harmful when the alias is close to an expected shear wave arrival time.
One way to address the problem of aliasing is to move the acoustic receivers closer together on the downhole measurement tool. Changing the spacing in the array of receivers alters the aliasing effect. In general, the smaller the receiver spacing, the farther the alias tends to be moved away from a potential shear wave arrival. However, reducing the receiver spacing also reduces array coverage unless more receivers are added. Moreover, a reduction in receiver array coverage, which increases the uncertainty in the coherence slowness analysis. Additional receivers to mitigate the reduction in array coverage are known to increase the cost and complexity of the downhole tool.
Therefore, there exists a need for an improved downhole measurement tool that can be used for determining a shear wave velocity of a subterranean formation, and that addresses one or more of the shortcomings described above. In particular, it will be appreciated that a downhole measurement tool that reduces the aliasing effect without excessively reducing the overall receiver array coverage, or increasing the expense of the tool, would be highly advantageous, since many of the above stated disadvantages would thus be obviated.